Conventionally, in a process of manufacturing an automobile, a bore in a cylinder block in an engine is formed by cutting machining, and thereafter a cylinder head, a crankcase and the like are assembled to the cylinder block.
Here, since a piston held in the bore is formed in a cross section round shape, cutting machining is performed such that the cross-sectional shape of the bore is close to a circle.
However, even when the bore of the cylinder block is machined in a cross section round shape, the shape of the bore is deformed when the cylinder head, the crankcase and the like are assembled. The bore deforming as described above causes an increase in the sliding resistance between the bore and the piston at the time of using the engine, and thus it is likely that the engine cannot achieve desired performance.
Hence, when the bore of the cylinder block is machined, a dummy head imitating the cylinder head is attached to machine the bore and the dummy head is removed when the machining of the bore is completed.
However, when the dummy head and the like are attached and removed each time the bore of the cylinder block is machined, productivity is lowered disadvantageously and significantly.
In order to solve this problem, the following method is proposed (see Patent Documents 1 and 2). Specifically, the dummy head is first fitted to the cylinder block, and the bore is machined in a cross section round shape with a machine tool. Then, the dummy head is removed from the cylinder block. Then, since a stress caused by the assembly of the dummy head is eliminated, the shape of the bore is deformed, and its cross section is formed in a non-round shape. The overall shape of the bore which is formed in a cross section non-round shape is measured, and the NC data thereof is generated.
Specifically, with respect to the NC data, on the bore from which the dummy head is removed and which is formed in a cross section non-round shape, measurement points are set at predetermined intervals along the axis line of the bore, and the cross-sectional shape of the bore is measured at each of the measurement points.
Thereafter, based on the generated NC data, boring machining is performed, without the dummy head being fitted, on the un-machined cylinder block, and thus a bore in a non-round shape is formed.
In this way, even when the bore is machined without the dummy head being attached to the cylinder block, the bore is formed in a round shape when the cylinder head is fitted.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2007-313619
Patent Document 2: PCT International Publication No. WO2009/125638